unit: unit type dependent status messages

unit: unit type dependent status messages

Instead of generic "Starting..." and "Started" messages for all unit use
type-dependent messages. For example, mounts will announce "Mounting..."
and "Mounted".

Add status messages to units of types that used to be entirely silent
(automounts, sockets, targets, devices). For unit types whose jobs are
instantaneous, report only the job completion, not the starting event.
Socket units with non-instantaneous jobs are rare (Exec*= is not used
often in socket units), so I chose not to print the starting messages
for them either.

This will hopefully give people better understanding of the boot.